1. Field of the Invention
The present invention relates to a learning device that learns a control parameter, which is used for deciding a control content of a controlled object, in relation to a criterion variable.
2. Description of Related Art
As an example of this kind of learning device, there is a device that measures an injection start delay td since an injection command is outputted to an injector of an internal combustion engine until injection is actually performed and that performs learning by storing and updating a measurement value of the injection start delay td as a control parameter of each injector (for example, refer to Patent document 1: JP-A-2009-57924). Timing for outputting the injection command and the like are controlled by using the delay td stored and updated in this way.
The above-described delay td takes different values according to fuel pressure supplied to the injector (i.e., fuel pressure as of injection start timing). Therefore, the inventors of the present invention examined learning the delay td (control parameter) in relation to the fuel pressure (criterion variable) as follows. In an example of FIG. 7A, delays td(30), td(50), td(80) corresponding to specific values of the fuel pressure (30 MPa, 50 MPa, 80 MPa in example of FIG. 7A) are stored and updated as learning values. For example, when the measured delay is indicated by a point A0 in FIG. 7A, the learning value td(50) closest to the point A0 is updated by linear interpolation. More specifically, an intersection point between a straight line L, which connects the learning value td(30) and the measurement value A0, and the fuel pressure of 50 MPa is used as the updated learning value td(50).
However, if a characteristic expressing a relationship between the fuel pressure and the delay td is not a straight line but a curve line (refer to curve line R in FIG. 7B) and points A1, A2, A3 on the curve line R are measured repeatedly, the learning value td(50) is updated sequentially from B1 to B2 and then to B3 when the above-mentioned linear interpolation is performed. That is, the learning value td(50) repeats increase or decrease and hunts. Even if the characteristic expressing the relationship between the fuel pressure and the delay td is the straight line, the learning value td(50) provided by the linear interpolation based on the learning value td(30) also hunts when the present learning value td(30) has deviated from a true characteristic value.